Reactions of phenolic compounds and sulfur halides, in general, to form a broad spectrum of the reaction products too numerous to mention, are well-known in the prior art. Many of these prior art reactions provide for the formation of a mixture of reaction products. However, others require the reaction of specific phenols and sulfur halides while maintaining a myriad of reaction conditions, to form specific thiophenolic materials. More specifically, the product recovered on contacting a phenol and sulfur dichloride generally contains a combination of mono- and poly- sulfides, chlorinated phenols, sulfonium compounds, polymeric materials, and the like. However, if a specific 4,4'-monothiodiphenolic compound, such as 4,4'-thiodiphenol, were the desired product, any other materials present would be considered by-products and would require separation from the total material produced and recovered. Recovery is a particularly acute problem in the case of the polysulfide by-products since they are formed as part of the solid, crystalline product phase along with the desired 4,4'-monothiodiphenolic product. Thus, when polysulfides are present, further recovery steps are necessary to separate the respective solid phases. Unlike their monothiophenolic counterparts, polysulfides contain relatively weak sulfur-to-sulfur bonds. Therefore, on polymerization of these polysulfides, the resultant polymeric product will be relatively weak because of this defective bonding characteristic.
The yield of specific monothiodiphenols formed by the direct contact of a phenolic compound and a sulfur halide, according to the prior art processes, in cases where the formation of specific materials is desired, is relatively poor. And, in addition to the aforementioned drawbacks, known processes teach that relatively long contact times are generally required for specific product formation, in relatively poor yields. Furthermore, due to the relatively long contact times exhibited by the prior art processes, difficulties will result in conducting a continuous reaction, in an efficient manner, to selectively form high yields of specific 4,4'-monothiodiphenols. Moreover, the economic feasibility of such a process is highly questionable.
Many of the aforementioned phenolic reactions have been employed in preparing either stabilizers or antioxidants. For example, U.S. Pat. No. 1,849,489 to Howland describes the formation of a class of chemical compounds "adapted to retard deterioration of rubber." These compounds include sulfides of phenol. An example of this reaction is shown on page 2, beginning on line 55, wherein a relatively high concentration of phenols is reacted with a sulfur chloride compound in a chloroform solvent. A low yield of phenol sulfides was seemingly formed after a 2-hour reaction period. Furthermore, any combination of sulfides, polysulfides or isomers thereof provides an acceptable reaction product for use therein. A high specific product yield is not required in the above formation process.
Another process which describes the production of sulfur-containing phenols useful as antioxidants is found in U.S. Pat. No. 3,678,115 to Fujisawa et al. In this case, the production of a sulfur-containing sterically-hindered phenol, i.e., 4,4'-thiobis(2,6-di-t-butyl phenol) is disclosed. The formation of the above sterically-hindered phenolic product is conducted according to an entirely different reaction system than is provided for its nonhindered counterpart. The Fujisawa process, for instance, includes reacting a high concentration (greater than 25% by weight) of 2,6-di-t-butyl phenol (a hindered phenolic reactant) employing either sulfur chloride or sulfur dichloride, and requiring, in the sulfur dichloride reaction, a reaction period of greater than about 24 hours. The use of relatively high concentrations of phenol causes additional problems in mass transport of the product mixture formed, thereby further increasing the total reaction time and, in case of reaction of a phenol and sulfur dichloride, promotes the formation of a noncrystalline phase in the reaction system which is detrimental to the facilitation of high yields of the desired product. Example 1 of Fujisawa indicates that the 4,4'-thiobis(2,6-di-t-butyl phenol) was present in only a 23% yield after an initial 18-hour reaction period, 53% of the total product formed being polysulfides. Thereafter, in order to provide a higher yield of the monothiophenolic product, the polythiobisphenols present in the mixture were further reacted for about 14 hours with a strong base in order to cleave the sulfur bridges to form of mercapto compound which, in turn, recombines with the unreacted phenolic reactant to form additional amounts of the desired 4,4'-thiobis product.
U.S. Pat. No. 2,139,766 to Mikeska et al. provides for the formation of a dialkyl diphenol thio ether compound by the relatively limited reaction of a high concentration of phenol with sulfur dichloride in a carbon disulfide solvent. The product, which is then added to a mineral lubricating oil, acts as an antioxidant. As previously discussed with respect to the above cited patents, when the end use of a thiodiphenolic product is as an antioxidant, selectivity in forming a high yield monothiodiphenolic product, having a minimum amount of by-products, is not required.
In another prior art method (see U.S. Pat. No. 3,296,310 to Gilbert), the process for producing thiobis phenols is conducted by reacting elemental sulfur with the phenol in the presence of a halogen. The process is claimed by the inventor as an improvement over methods which employ the reaction of phenol with sulfur halide, such as sulfur chloride or sulfur dichloride. In this latter case, the halogen employed as a catalyst is either iodine or bromine, not chlorine. Selectivity, yield and reactor time problems previously expressed are again present in the Gilbert process. In fact, many of the aforementioned problems are further magnified since a mixture of thiobis phenols are formed in which the sulfur atoms are attached to the phenol in either the ortho- or para- positions, as opposed to selectively providing a predominant attachment in only the para- position.
U.S. Pat. No. 2,425,824 to Peters et al. provides a process in which sulfur halides are reacted with a high concentration of a phenolic compound, in a mole ratio of from about 1.25 to 1.75:2.0, and preferably 1.5:2.0. Poor selectivity and yield are evident on examining the product material. The products formed are continuously discharged from the reactor. The use of sulfur monochloride, sulfur dichloride, or mixtures thereof, respectively, as the reactant herein, is shown to be equivalent. As described in column 3, beginning at line 2, "the important point of the process is to at all points maintain the desired narrow limits of ratios of the reactants . . . " Therefore, one would conclude that only by employing the above specified reactant ratio could a continuous phenolic-sulfur chloride reaction process be maintained.
U.S. Pat. No. 3,057,926 to Coffield relates to the production of an antioxidant wherein a substituted phenol is reacted with either sulfur monochloride or sulfur dichloride to produce a substituted thio bis phenol compound having one or more sulfur atoms attached to the respective rings. Stoichiometric amounts, 2:1 mole ratios of phenol per mole of sulfur halide, and high concentrations of phenol are employed in the Coffield process. The previously enumerated problems associated with selectivity, yield, reaction time, and high concentration are once more present herein. In addition, the patent does not distinguish between the use of the respective sulfur halides as reactants. Finally, complicated and arduous addition techniques are provided in the examples of Coffield in order to produce, as in Example 2, a thio bis phenolic product.
Finally, U.S. Pat. No. 3,390,190 to Curtis et al. provides a process for purifying a conventionally produced thiophenolic product prepared by the reaction of phenol and sulfur dichloride in toluene. The final product is obtained by refluxing the initial crude reaction mixture, separating a tar phase, and purifying the crude product so obtained by dissolving it in an alkaline solution of a weak, inorganic base, such as sodium carbonate. Curtis provides a 4,4'-dihydroxy diphenyl sulfide material produced, as illustrated in Example 1, in a 55.2% yield based on theory. Furthermore, after purification thereof, the actual yield based on starting materials of the final purified product is about 47%, due to losses in product incurred during the above purification. And, even after performing these elaborate purification steps, which require a substantial period of time to complete, the amount of polysulfides present in the final 4,4'-isomeric product recovered (see Tables III-IV) is still 2.4 -3.5% by weight.